The present invention relates to decorative lighting chains and more particularly, to an improved structure of LED lighting chain, which provides a constant brightness that is free from the limitation of the number of LEDs and, which is safe and durable in use.
FIG. 1 is a schematic circuit diagram of a conventional decorative lighting chain. According to this prior art design, the decorative lighting chain comprises a front plug 61, a rear plug 62, an AC-DC converter 7, and at least one LED serial set 8. Each LED serial set 8 is comprised of a plurality of LEDs connected in series. A power cable is installed to connect the positive and negative terminals of the front plug 61 and the rear plug 62. The AC-DH converter 7 is connected in parallel to the power cable. Referring also to FIG. 2, the AC-DC converter 7 is a bridge rectifier having four wires 91, 92, 93 and 94. Due to the limitation of the four wires 91, 92, 93 and 94 of the AC-DC converter 7, the decorative lighting chain is less attractive. Further, the LED serial set 8 is not provided with a voltage dropping device to stabilize the voltage at the two ends of the LED serial set 8. Because the rated voltage of a regular LED is 2.2V, the number of LEDs of the LED serial set 8 is limited.
U.S. Pat. No. 6,972,528B2 uses only three wires, thereby having a better sense of beauty. Further, this design has a voltage-reducing device, which comprises a resistor in series with a capacitor. By means of selecting a different resistance value, the voltage-reducing device fits a different number of LEDs. However, this structure for LED lighting chain still has drawbacks as follows:
1. Because this structure of LED lighting chain works at a constant voltage working, its input voltage range is limited. When working at AC 120V, the number of the LEDs must not exceed by 60, otherwise the brightness will be insufficient.
2. It adopts built-in resistance load to fit different LED serial sets of different numbers of LEDs, resulting high consumption of load power and low efficiency.
3. It causes a high voltage pulse during operation, and the LEDs may be damaged easily during the starting transient.
4. The working stability of this structure of LED lighting chain is low. Further, the LED lighting chain tends to be interfered with external noises, and the control of its LED brightness is not easy.
The present invention has been accomplished under the circumstances in view. According to one aspect of the present invention, the LED lighting chain has a resistance and two capacitors installed in the LED serial set thereof to constitute a voltage dropping device. By means of changing the capacitors and the resistance, the voltage of the LED serial set is relatively changed, i.e., increasing the number of the LEDs does not change the brightness of the LED serial set. According to another aspect of the present invention, a resistance is connected in parallel to each capacitor of the voltage dropping device, so that when power is off, residual electric energy is discharged out of each capacitor to assure safety use of the LED lighting chain. According to still another aspect of the present invention, when starting the LED lighting train, the electric current is stable. Even if a transient high voltage is produced due to an electronic shock, the voltage will soon be stabilized, preventing damage to the LEDs, and therefore the working life of each LED is prolonged.